In the offshore production of oil, gas, and other production fluids, floating vessels are frequently used to transport the production fluids to onshore consuming markets. The production fluids are produced from the deck of an offshore structure which is firmly anchored to the sea floor with pilings. To convey production fluids from a subsea well to the water surface, the offshore structure supports a fluid-carrying system called a riser. A flowline connected to the upper end of the riser conveys the production fluids to a storage tanker which may be temporarily or permanently moored to the offshore structure. The production fluids are offloaded from the storage tanker to shuttle tankers for transportation to an onshore market.
To moor a floating vessel in a water environment, a mooring system should be sufficiently flexible to accommodate movement of the vessel relative to the fixed offshore structure. As the moored vessel is acted upon by loading forces induced by wind, waves, ice, and ocean currents, the vessel will roll, pitch and heave. In addition, the vessel will yaw about its mooring point as the direction of the loading forces varies. If the mooring system is rigid, movement of the vessel due to loading forces could damage the mooring system. If the movement of the vessel is sufficiently great, the offshore structure could also be damaged.
To furnish a flexible mooring system in an open water environment, nylon hawsers are often used to moor a vessel to an offshore structure. Because nylon is an elastic material, nylon hawsers dampen the movement of the vessel which is caused by the loading forces. Although nylon hawsers are sufficiently strong to moor a storage vessel to an offshore structure in an open water environment, nylon hawsers cannot be used year-round in a cryogenic environment such as the Arctic. During the Arctic winter, nylon loses its resiliency and becomes brittle. This brittleness reduces the breaking strength of nylon and may lead to failure of a nylon hawser. The presence of moving pack ice containing ice ridges up to thirty feet in height also prevents nylon hawsers from being used during the Arctic winter. Nylon hawsers which are weakened by the cold are particularly susceptible to failure as the moving pack ice acts against the moored vessel.
To handle forces induced by moving pack ice, rigid mooring chains are typically used to moor a vessel to an offshore structure during the Arctic winter. However, rigid mooring chains are not suitable in an open water environment because they are not sufficiently elastic to accommodate movement of the storage vessel. As the vessel moves toward the structure, the mooring chains can become slack. As loading forces urge the vessel away from the structure, the vessel can gain sufficient momentum to impart a large impact force to the mooring chains and offshore structure when the vessel reaches the excursion length of the chains. In such event, the chains and structure may be damaged.
Although nylon hawsers can moor a vessel in an open water environment and rigid mooring chains can be substituted for the nylon hawsers during the winter months, valuable production time is lost when the fluid-carrying system is shut down to convert from nylon hawsers to the mooring chains. The lost production time may be particularly significant during the spring and fall seasons when the vessel may alternately be exposed to moving pack ice and to open water. Therefore, there is a need for a mooring system that can accommodate loading forces acting on a vessel due to wind, waves, ice, or ocean currents. Furthermore, there is a need for a mooring system that can be used in a water environment and in an ice-bound environment.